A variety of technologies for the capture of particulate emissions from diesel engines are presently available. Diesel engines have long been the engines of choice for powering heavy duty equipment such as trucks, trains, tractors and construction vehicles, since diesel engines generally utilize lesser amounts of fuel to generate power comparable to those of gasoline engines of equivalent size and weight. Particulate (e.g. soot) emissions from diesel engines can be relatively heavy, however, and accordingly devices such as soot traps and soot combustors have been designed to help capture and eliminate soot particulates.
In comparison with diesel engines and fuels, gasoline is typically more completely combusted during ordinary gasoline engine operation, with the primary polluting constituents of gasoline engine exhaust streams including volatile unburned hydrocarbons, carbon monoxide, and nitrogen oxides. However, fuel combustion is somewhat different in the case of gasoline direct injection (GDI) engines, as evidenced by the fact that the direct injection of the fuel into the combustion chambers of such engines can result in somewhat larger concentrations of unburned particulates in the engine exhaust stream.
Particulate emissions from sources such as gasoline-direct-injection engines can present a somewhat more difficult pollution control problem than diesel engine emissions. That is because the particulates generated during the operation of such engines predominantly fall in the sub-micrometer (e.g., nanometer) size range. Conventional diesel traps may not be well adapted for the capture and elimination of such particulates. For example, ceramic filters similar in configuration to current wall-flow diesel particulate filters (DPFs) may be made fine enough in porosity to capture nanometer-sized particles, but in that case may be so high in gas flow resistance as to raise engine exhaust back-pressures and reduce engine fuel efficiencies to objectionable levels. Other soot-handling systems, such as soot combustors and electrostatic precipitators designed to trap and/or combust soot particles of conventional size, are too inefficient or too costly and complex for widespread use in motor vehicles.